[0.64] Add JSI.MemoryMappedScriptStore runtime option to control usag…

…e of MemoryMappedBuffer (#7957)

* Add JSI.MemoryMappedScriptStore runtime option to control usage of MemoryMappedBuffer (#7943)

* Condition using memory mapped buffer to runtime option JSI.MemoryMappedScriptStore

* Change files

* Remove change file

* Change files

change/react-native-windows-749550b6-6f9d-4baa-a66d-1bd43d055421.json

@@ -0,0 +1,7 @@
+{
+  "type": "patch",
+  "comment": "Add JSI.MemoryMappedScriptStore runtime option to control usage of MemoryMappedBuffer (#7943)",
+  "packageName": "react-native-windows",
+  "email": "julio.rocha@microsoft.com",
+  "dependentChangeType": "patch"
+}

vnext/Desktop.UnitTests/ScriptStoreTests.cpp

@@ -3,6 +3,7 @@
 
 #include <BaseScriptStoreImpl.h>
 #include <CppUnitTest.h>
+#include <RuntimeOptions.h>
 
 // Windows API
 #include <Windows.h>
@@ -23,6 +24,10 @@ using winrt::Windows::System::Diagnostics::ProcessDiagnosticInfo;
 namespace Microsoft::JSI::Test {
 
 TEST_CLASS (ScriptStoreIntegrationTest) {
+  TEST_CLASS_INITIALIZE(Init) {
+    React::SetRuntimeOptionBool("JSI.MemoryMappedScriptStore", true);
+  }
+
   // Do not run this test in parallel with others.
   // It uses process telemetry and should run on isolation.
   TEST_METHOD(RetrievePreparedScriptMemoryUsage) {
@@ -56,9 +61,11 @@ TEST_CLASS (ScriptStoreIntegrationTest) {
     auto endWorkingSet =
         ProcessDiagnosticInfo::GetForCurrentProcess().MemoryUsage().GetReport().WorkingSetSizeInBytes();
 
-    // Without memory mapping: about 4.25 MB (fileSize + overhead)
-    // Expected working set delta: 0x047000 bytes, about 0.28 MB (6.9% of the 4MB file size).
-    Assert::IsTrue(endWorkingSet - startWorkingSet < 0.3 * 1024 * 1024);
+    // Based on recurring local testing:
+    // Without memory mapping: about 6.11 MB (fileSize + app overhead)
+    // With memory mapping: about 2.14 MB (view overhead + app overhead)
+    // Expected working set size should be lower than the actual file size, provided it is larger than the app overhead
+    Assert::IsTrue(endWorkingSet - startWorkingSet < fileSize);
   }
 };
 } // namespace Microsoft::JSI::Test

vnext/Shared/BaseScriptStoreImpl.cpp

@@ -6,6 +6,8 @@
 #include "BaseScriptStoreImpl.h"
 #include "MemoryMappedBuffer.h"
 
+#include <RuntimeOptions.h>
+
 // C++/WinRT
 #include <winrt/base.h>
 
@@ -144,7 +146,26 @@ std::unique_ptr<const jsi::Buffer> LocalFileSimpleBufferStore::getBuffer(const s
     std::terminate();
   }
 
-  return Microsoft::JSI::MakeMemoryMappedBuffer(winrt::to_hstring(storeDirectory_ + bufferId).c_str());
+  if (Microsoft::React::GetRuntimeOptionBool("JSI.MemoryMappedScriptStore")) {
+    return Microsoft::JSI::MakeMemoryMappedBuffer(winrt::to_hstring(storeDirectory_ + bufferId).c_str());
+  } else {
+    // Treat buffer id as the relative path fragment.
+    std::ifstream file(storeDirectory_ + bufferId, std::ios::binary | std::ios::ate);
+
+    if (!file) {
+      return nullptr;
+    }
+
+    std::streamsize size = file.tellg();
+    file.seekg(0, std::ios::beg);
+
+    auto buffer = std::make_unique<ByteArrayBuffer>(static_cast<size_t>(size));
+    if (!file.read(reinterpret_cast<char *>(buffer->data()), size)) {
+      return nullptr;
+    }
+
+    return buffer;
+  }
 }
 
 bool LocalFileSimpleBufferStore::persistBuffer(